1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element.
2. Description of the Related Art
Phase-change memory is a scalable, high speed, and non-volatile memory technology. It is targeted typically for mobile devices which require low power consumption. A phase-change memory cell must therefore provide low programming current, avoidance of high voltages, smaller cell size, faster phase transformation speed, and low cost. These requirements are difficult to meet given the current state of the art.
The most straightforward way to reduce the programming current is to reduce the heating area. A benefit of this strategy is simultaneous reduction of cell size. However, reducing the area results in a higher cell resistance, which increases the required driving voltage. All other considerations being the same, the amount of Joule heating is conserved, meaning the operating voltage is inversely proportional to the programming current. This is clearly not desirable. Reducing heating area does not necessarily improve other performance features. Phase transformation speed requires good thermal uniformity within the active region of the cell.
In reality, cooling becomes significant for smaller structures, and loss to environment becomes more important with increased surface/volume ratio. As a result, temperature uniformity is degraded. In addition, the required current density must increase as heating area is reduced. This poses an electromigration concern for reliability. Hence, it is important to not only reduce the current, but also required heating. Since the amount of Joule heating input is reduced, heat loss to the environment must be reduced even further.
The heating loss is proportional to the thermal conductivity of the surrounding dielectric material. As a reference, the thermal conductivity of a commonly used phase-change chalcogenide, Ge2Sb2Te5, is experimentally measured to have a range of values, averaging around a value of 0.3 W/m-K. The low conductivity is due to both low electron density and vacancies in the microstructure which enhance phonon scattering. Since it is the active material, it obviously cannot serve as the surrounding dielectric. Silicon nitride and silicon oxide are stable in contact with the chalcogenide. However, their thermal conductivities approach and sometimes exceed 1 W/m-K, which prohibits scaling down the programming current beyond the current state of the art.
One solution uses a mixture of the low thermal conductivity chalcogenide material with a stable higher thermal conductivity dielectric, such that the effective thermal conductivity of the mixture approaches that of the chalcogenide.
U.S. Pat. No. 5,933,365 “Memory element with energy control mechanism” discloses the use of thermal isolation layers which at least partially encapsulate the phase-change material. However, the selection of candidate materials far exceeds the range of materials available for state-of-the-art memory cell fabrication, and do not reflect the currently known thermal conductivities of such materials.
Therefore, it is necessary to develop a phase-change memory to solve the previously described problems.